Oxidative Degradation of Organic Contaminants by FeS in the Presence of O2

Reductive transformation of organic contaminants by FeS in anoxic environments has been documented previously, whereas the transformation in oxic environments remains poorly understood. Here we show that phenol can be efficiently oxidized in oxic FeS suspension at circumneutral pH value. We found that hydroxyl radicals (•OH) were the predominant reactive oxidant and that a higher O2 content accelerated phenol degradation. Phenol oxidation depended on •OH production and utilization efficiency, i.e., phenol degraded per •OH produced. Low FeS contents (≤1 g/L) produced less •OH but higher utilization efficiency, while high contents produced more •OH but lower utilization efficiency. Consequently, the most favorable conditions for phenol oxidation occurred during the long-term interaction between dissolved O2 and low levels of FeS (i.e., ≤1 g/L). Mössbauer spectroscopy suggests that FeS oxidation to lepidocrocite initially produced an intermediate Fe(II) phase that could be explained by the apparent preferential oxidation of structural S(−II) relative to Fe(II), rendering a higher initial •OH yield upon unit of Fe(II) oxidation. Trichloroethylene can be also oxidized under similar conditions. Our results demonstrate that oxidative degradation of organic contaminants during the oxygenation of FeS can be a significant but currently underestimated pathway in both natural and engineered systems.